The nice thing about MHTML is that compared to just a ZIP of files it is fully plain text and includes HTTP headers so that everything can be rendered the exact

The nice thing about MHTML is that compared to just a ZIP of files it is fully plain text and includes HTTP headers so that everything can be rendered the exact same way a browser would when served from a server. The headers can store additional metadata for each resource. It's also guaranteed to never go away because it uses the exact same way emails are encoded.